As current mobile communication systems evolve and new communications systems are developed, there is continuing demand for more powerful and efficient power amplifiers that are capable of operating over broader frequency ranges. Many of these communication systems employ mobile devices and access points, such as base stations, that are battery powered. For such communication devices, more efficient power amplifiers yield longer operating times between battery charges.
Further, the transmit power levels for mobile devices and especially access points are continuing to increase at the same time that sizes of these devices are shrinking. As the power levels increase, the amount of heat that is generated during amplification generally increases. Therefore, designers are faced with dissipating greater quantities of heat from shrinking communication devices or reducing the amount of heat generated by the power amplifiers therein. More efficient power amplifiers are preferred because they generate less heat than less efficient power amplifiers at corresponding power levels, and thus reduce the amount of heat to dissipate during operation.
Given the ever increasing demand for efficiency, the Doherty amplifier has become a popular power amplifier in mobile communication applications, especially base station applications. While relatively efficient compared to its rivals, the Doherty amplifier has a relatively limited bandwidth of operation. For example, a well-designed Doherty amplifier may provide an instantaneous bandwidth of 5 percent, which corresponds to about 100 MHz for a 2 GHz signal and is generally sufficient to support a single communication band. For example, Universal Mobile Telecommunications Systems (UMTS) devices operate in a band between 2.11 and 2.17 GHz, and thus require an instantaneous bandwidth of 60 MHz (2.17 GHz-2.11 GHz). A Doherty amplifier can be configured to support an instantaneous bandwidth of 60 MHz for the UMTS band. Accordingly, for communication devices that only need to support a single communication band, the limited operating bandwidth of the Doherty power amplifier poses no problems.
However, modern communication devices are often required to support various communication standards that employ different modulation techniques over a wide range of operating frequencies. These standards include but are not limited to the Global System for Mobile Communications (GSM), Personal Communication Service (PCS), Universal Mobile Telecommunications Systems (UMTS), Worldwide Interoperability for Microwave Access (WiMAX), Long Term Evolution (LTE), and the like.
The bands of operation for these standards range from around 800 MHz to 4 GHz for consumer telecommunication applications and from 20 MHz to 6 GHz for military applications. The GSM standards alone employ bands ranging from around 800 MHz to 2 GHz. For example, GSM-850 uses an 824-894 MHz band, GSM-900 uses an 890-960 MHz band, GSM-1800 uses a 1710-1880 MHz band, and GSM-1900 uses an 1850-1990 MHz band. UMTS uses a 2.11-2.17 GHz band. LTE uses a 2.6-2.7 GHz band, and WiMAX uses bands centered about 2.3, 2.5, 3.3 and 3.5 GHz. Thus, for devices that need to support multiple communication bands, a single Doherty amplifier is not sufficient.
For communication devices that support multiple standards over disparate communication bands, designers often employ multiple power amplifier chains for each of the different communication bands, which increases the size, cost, and complexity of the communication devices. As such, there is a need to increase the effective operating range of a Doherty power amplifier to support multiple communication bands, which are spread over a significant frequency range, while maintaining the efficiency afforded by current Doherty power amplifier designs.